In the parent application Ser. No. 07/997,306, a method of producing chlorine dioxide, suitable for use in a minimum adverse environmental impact pulp mill such as shown in grandparent application Ser. No. 07/922,334, from bleach plant effluents is disclosed. According to a primary aspect of that disclosure, chlorine-containing bleach plant effluent is evaporated or otherwise concentrated to a consistency great enough for incineration, and then it is incinerated, and the ash either used directly for chlorine dioxide production right on site, or after storage transportation of the ash to a chlorate manufacturing location off-site where chlorates are produced for eventual return to the pulp mill. According to the present invention, a method of acting on bleach plant effluents to produce chlorates, and ultimately typically chlorine dioxide, is provided that provides a number of important modifications of the general concept disclosed in the parent application, to provide enhanced versatility to the pulp mill while providing minimum adverse environmental impact.
According to the present invention, automatically and sequentially liquid effluents containing chlorides from a bleach plant are concentrated to a solids concentration level high enough for sodium chloride removal or for economic transport to a site off the mill premises. For example, concentration to a solids consistency of about 25-35% (e.g. 33%) is preferred. Then at least a portion of the concentrated liquid effluents is transported to a site off the mill premises, and at a chlorate production location off the mill site chlorate is produced from the transported material. Chlorates are then transported from the chlorate production location to the mill, or otherwise utilized.
The method according to the present invention may comprise the further step, between the concentrating and transporting steps, of storing the concentrated liquid effluents, and/or treating the concentrated liquid effluents to remove sodium chloride (e.g. by evaporative crystallization), so that only NaCl is transported to the chlorate production site. In the latter case, the concentrating step--preferably practiced by evaporation, e.g. using falling film metal-plastic laminate evaporator elements, each laminate having a thickness of about 100 microns or less--is practiced to produce a liquid stream having a high enough solids consistency to be combusted in the mill recovery boiler, and after sodium chloride removal, the chloride-depleted stream is fed to the recovery boiler to be burned therein.
The chlorates returned from the off-site production location are typically used to manufacture chlorine dioxide used in the bleach plant (to bleach the pulp) directly on the plant site. The chlorate production location also optionally produces acid and caustic, which also may be transported to the mill and used therein.
It is the primary object of the present invention to provide an effective, versatile method of handling bleach plant effluents so as to provide minimum adverse environmental impact, typically for the production of chlorine dioxide therefrom. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.